On Wed, May 22, 2019 at 07:54:57PM +0100, Jiong Wang wrote:
> eBPF ISA specification requires high 32-bit cleared when low 32-bit
> sub-register is written. This applies to destination register of ALU32 etc.
> JIT back-ends must guarantee this semantic when doing code-gen. x86_64 and
> AArch64 ISA has the same semantics, so the corresponding JIT back-end
> doesn't need to do extra work.
>
> However, 32-bit arches (arm, x86, nfp etc.) and some other 64-bit arches
> (PowerPC, SPARC etc) need to do explicit zero extension to meet this
> requirement, otherwise code like the following will fail.
>
> u64_value = (u64) u32_value
> ... other uses of u64_value
>
> This is because compiler could exploit the semantic described above and
> save those zero extensions for extending u32_value to u64_value, these JIT
> back-ends are expected to guarantee this through inserting extra zero
> extensions which however could be a significant increase on the code size.
> Some benchmarks show there could be ~40% sub-register writes out of total
> insns, meaning at least ~40% extra code-gen.
>
> One observation is these extra zero extensions are not always necessary.
> Take above code snippet for example, it is possible u32_value will never be
> casted into a u64, the value of high 32-bit of u32_value then could be
> ignored and extra zero extension could be eliminated.
>
> This patch implements this idea, insns defining sub-registers will be
> marked when the high 32-bit of the defined sub-register matters. For
> those unmarked insns, it is safe to eliminate high 32-bit clearnace for
> them.
>
> Algo:
> - Split read flags into READ32 and READ64.
>
> - Record index of insn that does sub-register write. Keep the index inside
> reg state and update it during verifier insn walking.
>
> - A full register read on a sub-register marks its definition insn as
> needing zero extension on dst register.
>
> A new sub-register write overrides the old one.
>
> - When propagating read64 during path pruning, also mark any insn defining
> a sub-register that is read in the pruned path as full-register.
>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
> Signed-off-by: Jiong Wang <jiong.wang@xxxxxxxxxxxxx>
> ---
> include/linux/bpf_verifier.h | 14 +++-
> kernel/bpf/verifier.c | 175 +++++++++++++++++++++++++++++++++++++++----
> 2 files changed, 173 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 1305ccb..60fb54e 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -36,9 +36,11 @@
> */
> enum bpf_reg_liveness {
> REG_LIVE_NONE = 0, /* reg hasn't been read or written this branch */
> - REG_LIVE_READ, /* reg was read, so we're sensitive to initial value */
> - REG_LIVE_WRITTEN, /* reg was written first, screening off later reads */
> - REG_LIVE_DONE = 4, /* liveness won't be updating this register anymore */
> + REG_LIVE_READ32 = 0x1, /* reg was read, so we're sensitive to initial value */
> + REG_LIVE_READ64 = 0x2, /* likewise, but full 64-bit content matters */
> + REG_LIVE_READ = REG_LIVE_READ32 | REG_LIVE_READ64,
> + REG_LIVE_WRITTEN = 0x4, /* reg was written first, screening off later reads */
> + REG_LIVE_DONE = 0x8, /* liveness won't be updating this register anymore */
> };
>
> struct bpf_reg_state {
> @@ -131,6 +133,11 @@ struct bpf_reg_state {
> * pointing to bpf_func_state.
> */
> u32 frameno;
> + /* Tracks subreg definition. The stored value is the insn_idx of the
> + * writing insn. This is safe because subreg_def is used before any insn
> + * patching which only happens after main verification finished.
> + */
> + s32 subreg_def;
> enum bpf_reg_liveness live;
> };
>
> @@ -232,6 +239,7 @@ struct bpf_insn_aux_data {
> int ctx_field_size; /* the ctx field size for load insn, maybe 0 */
> int sanitize_stack_off; /* stack slot to be cleared */
> bool seen; /* this insn was processed by the verifier */
> + bool zext_dst; /* this insn zero extends dst reg */
> u8 alu_state; /* used in combination with alu_limit */
> unsigned int orig_idx; /* original instruction index */
> };
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 95f93544..0efccf8 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -981,6 +981,7 @@ static void mark_reg_not_init(struct bpf_verifier_env *env,
> __mark_reg_not_init(regs + regno);
> }
>
> +#define DEF_NOT_SUBREG (-1)
> static void init_reg_state(struct bpf_verifier_env *env,
> struct bpf_func_state *state)
> {
> @@ -991,6 +992,7 @@ static void init_reg_state(struct bpf_verifier_env *env,
> mark_reg_not_init(env, regs, i);
> regs[i].live = REG_LIVE_NONE;
> regs[i].parent = NULL;
> + regs[i].subreg_def = DEF_NOT_SUBREG;
shouldn't it be moved into __mark_reg_unknown ?
Also what about my old suggestion to use DEF_NOT_SUBREG==0
to be on safer side when we zero things out?
> }
>
> /* frame pointer */
> @@ -1136,7 +1138,7 @@ static int check_subprogs(struct bpf_verifier_env *env)
> */
> static int mark_reg_read(struct bpf_verifier_env *env,
> const struct bpf_reg_state *state,
> - struct bpf_reg_state *parent)
> + struct bpf_reg_state *parent, u8 flag)
> {
> bool writes = parent == state->parent; /* Observe write marks */
> int cnt = 0;
> @@ -1151,17 +1153,26 @@ static int mark_reg_read(struct bpf_verifier_env *env,
> parent->var_off.value, parent->off);
> return -EFAULT;
> }
> - if (parent->live & REG_LIVE_READ)
> + /* The first condition is more likely to be true than the
> + * second, checked it first.
> + */
> + if ((parent->live & REG_LIVE_READ) == flag ||
> + parent->live & REG_LIVE_READ64)
> /* The parentage chain never changes and
> * this parent was already marked as LIVE_READ.
> * There is no need to keep walking the chain again and
> * keep re-marking all parents as LIVE_READ.
> * This case happens when the same register is read
> * multiple times without writes into it in-between.
> + * Also, if parent has the stronger REG_LIVE_READ64 set,
> + * then no need to set the weak REG_LIVE_READ32.
> */
> break;
> /* ... then we depend on parent's value */
> - parent->live |= REG_LIVE_READ;
> + parent->live |= flag;
> + /* REG_LIVE_READ64 overrides REG_LIVE_READ32. */
> + if (flag == REG_LIVE_READ64)
> + parent->live &= ~REG_LIVE_READ32;
> state = parent;
> parent = state->parent;
> writes = true;
> @@ -1173,12 +1184,111 @@ static int mark_reg_read(struct bpf_verifier_env *env,
> return 0;
> }
>
> +/* This function is supposed to be used by the following 32-bit optimization
> + * code only. It returns TRUE if the source or destination register operates
> + * on 64-bit, otherwise return FALSE.
> + */
> +static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
> + u32 regno, struct bpf_reg_state *reg, enum reg_arg_type t)
> +{
> + u8 code, class, op;
> +
> + code = insn->code;
> + class = BPF_CLASS(code);
> + op = BPF_OP(code);
> + if (class == BPF_JMP) {
> + /* BPF_EXIT for "main" will reach here. Return TRUE
> + * conservatively.
> + */
> + if (op == BPF_EXIT)
> + return true;
> + if (op == BPF_CALL) {
> + /* BPF to BPF call will reach here because of marking
> + * caller saved clobber with DST_OP_NO_MARK for which we
> + * don't care the register def because they are anyway
> + * marked as NOT_INIT already.
> + */
> + if (insn->src_reg == BPF_PSEUDO_CALL)
> + return false;
> + /* Helper call will reach here because of arg type
> + * check, conservatively return TRUE.
> + */
> + if (t == SRC_OP)
> + return true;
> +
> + return false;
> + }
> + }
> +
> + if (class == BPF_ALU64 || class == BPF_JMP ||
> + /* BPF_END always use BPF_ALU class. */
> + (class == BPF_ALU && op == BPF_END && insn->imm == 64))
> + return true;
> +
> + if (class == BPF_ALU || class == BPF_JMP32)
> + return false;
> +
> + if (class == BPF_LDX) {
> + if (t != SRC_OP)
> + return BPF_SIZE(code) == BPF_DW;
> + /* LDX source must be ptr. */
> + return true;
> + }
> +
> + if (class == BPF_STX) {
> + if (reg->type != SCALAR_VALUE)
> + return true;
> + return BPF_SIZE(code) == BPF_DW;
> + }
> +
> + if (class == BPF_LD) {
> + u8 mode = BPF_MODE(code);
> +
> + /* LD_IMM64 */
> + if (mode == BPF_IMM)
> + return true;
> +
> + /* Both LD_IND and LD_ABS return 32-bit data. */
> + if (t != SRC_OP)
> + return false;
> +
> + /* Implicit ctx ptr. */
> + if (regno == BPF_REG_6)
> + return true;
compiler will optimize above 'if' away.
What's the point of the above stmt?
> +
> + /* Explicit source could be any width. */
> + return true;
> + }
> +
> + if (class == BPF_ST)
> + /* The only source register for BPF_ST is a ptr. */
> + return true;
ditto
> +
> + /* Conservatively return true at default. */
> + return true;
> +}
> +
> +static void mark_insn_zext(struct bpf_verifier_env *env,
> + struct bpf_reg_state *reg)
> +{
> + s32 def_idx = reg->subreg_def;
> +
> + if (def_idx == DEF_NOT_SUBREG)
> + return;
> +
> + env->insn_aux_data[def_idx].zext_dst = true;
> + /* The dst will be zero extended, so won't be sub-register anymore. */
> + reg->subreg_def = DEF_NOT_SUBREG;
> +}
> +
> static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
> enum reg_arg_type t)
> {
> struct bpf_verifier_state *vstate = env->cur_state;
> struct bpf_func_state *state = vstate->frame[vstate->curframe];
> + struct bpf_insn *insn = env->prog->insnsi + env->insn_idx;
> struct bpf_reg_state *reg, *regs = state->regs;
> + bool rw64;
>
> if (regno >= MAX_BPF_REG) {
> verbose(env, "R%d is invalid\n", regno);
> @@ -1186,6 +1296,7 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
> }
>
> reg = ®s[regno];
> + rw64 = is_reg64(env, insn, regno, reg, t);
> if (t == SRC_OP) {
> /* check whether register used as source operand can be read */
> if (reg->type == NOT_INIT) {
> @@ -1196,7 +1307,11 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
> if (regno == BPF_REG_FP)
> return 0;
>
> - return mark_reg_read(env, reg, reg->parent);
> + if (rw64)
> + mark_insn_zext(env, reg);
> +
> + return mark_reg_read(env, reg, reg->parent,
> + rw64 ? REG_LIVE_READ64 : REG_LIVE_READ32);
> } else {
> /* check whether register used as dest operand can be written to */
> if (regno == BPF_REG_FP) {
> @@ -1204,6 +1319,7 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
> return -EACCES;
> }
> reg->live |= REG_LIVE_WRITTEN;
> + reg->subreg_def = rw64 ? DEF_NOT_SUBREG : env->insn_idx;
> if (t == DST_OP)
> mark_reg_unknown(env, regs, regno);
> }
> @@ -1383,7 +1499,8 @@ static int check_stack_read(struct bpf_verifier_env *env,
> state->regs[value_regno].live |= REG_LIVE_WRITTEN;
> }
> mark_reg_read(env, ®_state->stack[spi].spilled_ptr,
> - reg_state->stack[spi].spilled_ptr.parent);
> + reg_state->stack[spi].spilled_ptr.parent,
> + REG_LIVE_READ64);
> return 0;
> } else {
> int zeros = 0;
> @@ -1400,7 +1517,9 @@ static int check_stack_read(struct bpf_verifier_env *env,
> return -EACCES;
> }
> mark_reg_read(env, ®_state->stack[spi].spilled_ptr,
> - reg_state->stack[spi].spilled_ptr.parent);
> + reg_state->stack[spi].spilled_ptr.parent,
> + size == BPF_REG_SIZE
> + ? REG_LIVE_READ64 : REG_LIVE_READ32);
I don't think that's correct.
It's a parial read from a stack slot that doesn't contain a pointer.
It can be of any size and any offset.
Hence 4-byte read can read upper bits too.
> if (value_regno >= 0) {
> if (zeros == size) {
> /* any size read into register is zero extended,
> @@ -2109,6 +2228,12 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
> value_regno);
> if (reg_type_may_be_null(reg_type))
> regs[value_regno].id = ++env->id_gen;
> + /* A load of ctx field could have different
> + * actual load size with the one encoded in the
> + * insn. When the dst is PTR, it is for sure not
> + * a sub-register.
> + */
> + regs[value_regno].subreg_def = DEF_NOT_SUBREG;
because of cases like above I think that DEF_NOT_SUBREG==0 is a safer choice.
To make any mark_reg_* to set it DEF_NOT_SUBREG and only two places
would have to do dst_reg->subreg_def = env->insn_idx + 1;
> }
> regs[value_regno].type = reg_type;
> }
> @@ -2368,7 +2493,9 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
> * the whole slot to be marked as 'read'
> */
> mark_reg_read(env, &state->stack[spi].spilled_ptr,
> - state->stack[spi].spilled_ptr.parent);
> + state->stack[spi].spilled_ptr.parent,
> + access_size == BPF_REG_SIZE
> + ? REG_LIVE_READ64 : REG_LIVE_READ32);
> }
> return update_stack_depth(env, state, min_off);
> }
> @@ -3332,6 +3459,9 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
> check_reg_arg(env, caller_saved[i], DST_OP_NO_MARK);
> }
>
> + /* helper call returns 64-bit value. */
> + regs[BPF_REG_0].subreg_def = DEF_NOT_SUBREG;
> +
> /* update return register (already marked as written above) */
> if (fn->ret_type == RET_INTEGER) {
> /* sets type to SCALAR_VALUE */
> @@ -4263,6 +4393,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
> */
> *dst_reg = *src_reg;
> dst_reg->live |= REG_LIVE_WRITTEN;
> + dst_reg->subreg_def = DEF_NOT_SUBREG;
> } else {
> /* R1 = (u32) R2 */
> if (is_pointer_value(env, insn->src_reg)) {
> @@ -4273,6 +4404,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
> } else if (src_reg->type == SCALAR_VALUE) {
> *dst_reg = *src_reg;
> dst_reg->live |= REG_LIVE_WRITTEN;
> + dst_reg->subreg_def = env->insn_idx;
> } else {
> mark_reg_unknown(env, regs,
> insn->dst_reg);
> @@ -5352,6 +5484,8 @@ static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn)
> * Already marked as written above.
> */
> mark_reg_unknown(env, regs, BPF_REG_0);
> + /* ld_abs load up to 32-bit skb data. */
> + regs[BPF_REG_0].subreg_def = env->insn_idx;
> return 0;
> }
>
> @@ -6292,20 +6426,33 @@ static bool states_equal(struct bpf_verifier_env *env,
> return true;
> }
>
> +/* Return 0 if no propagation happened. Return negative error code if error
> + * happened. Otherwise, return the propagated bit.
> + */
> static int propagate_liveness_reg(struct bpf_verifier_env *env,
> struct bpf_reg_state *reg,
> struct bpf_reg_state *parent_reg)
> {
> + u8 parent_flag = parent_reg->live & REG_LIVE_READ;
> + u8 flag = reg->live & REG_LIVE_READ;
> int err;
>
> - if (parent_reg->live & REG_LIVE_READ || !(reg->live & REG_LIVE_READ))
> + /* When comes here, read flags of PARENT_REG or REG could be any of
> + * REG_LIVE_READ64, REG_LIVE_READ32, REG_LIVE_NONE. There is no need
> + * of propagation if PARENT_REG has strongest REG_LIVE_READ64.
> + */
> + if (parent_flag == REG_LIVE_READ64 ||
> + /* Or if there is no read flag from REG. */
> + !flag ||
> + /* Or if the read flag from REG is the same as PARENT_REG. */
> + parent_flag == flag)
> return 0;
>
> - err = mark_reg_read(env, reg, parent_reg);
> + err = mark_reg_read(env, reg, parent_reg, flag);
> if (err)
> return err;
>
> - return 0;
> + return flag;
> }
>
> /* A write screens off any subsequent reads; but write marks come from the
> @@ -6339,8 +6486,10 @@ static int propagate_liveness(struct bpf_verifier_env *env,
> for (i = frame < vstate->curframe ? BPF_REG_6 : 0; i < BPF_REG_FP; i++) {
> err = propagate_liveness_reg(env, &state_reg[i],
> &parent_reg[i]);
> - if (err)
> + if (err < 0)
> return err;
> + if (err == REG_LIVE_READ64)
> + mark_insn_zext(env, &parent_reg[i]);
> }
>
> /* Propagate stack slots. */
> @@ -6350,11 +6499,11 @@ static int propagate_liveness(struct bpf_verifier_env *env,
> state_reg = &state->stack[i].spilled_ptr;
> err = propagate_liveness_reg(env, state_reg,
> parent_reg);
> - if (err)
> + if (err < 0)
> return err;
> }
> }
> - return err;
> + return 0;
> }
>
> static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> --
> 2.7.4
>
